It has been widely known to use a film of plasticized polyvinyl butyral as an interlayer for bonding the individual glass sheets of a laminated safety glass. The plasticized polyvinyl butyral film is widely used in safety glass in automobiles, air planes and building materials because of its high adhesiveness and superior light stability, transparency and low-temperature flexibility.
The surface of the plasticized polyvinyl butyral film, however, is very tacky, and presents a problem of blocking at the time of windup after film formation. It is the current practice to prevent blocking by embossing the surface of the film, or spraying an antiblocking agent such as sodium hydrogen carbonate on the surface of the film. However, this brings about the defect that when glasses are to be bonded through the polyvinyl butyral film, the film must go through the steps of washing and drying for removal of the antiblocking agent before use.
Use of a plasticized polyvinyl chloride film is effective for preventing blocking of the interlayer film. But since this film is not adhesive to glass, it is useless as an interlayer of laminated safety glass.
It is known to use a film of a vinyl chloride/glycidyl methacrylate copolymer containing about 40% by weight of a plasticizer for the purpose of reducing adhesiveness. Japanese Laid-Open Patent Publication No. 121016/77 states that the bond strength between this interlayer film and a glass sheet is, for example, 4 to 10 pummel units. However, the interlayer film of laminated safety glass shown in FIG. 3 of this Japanese Laid-Open Patent Publication lacks penetration resistance. When the content of glycidyl methacrylate in the copolymer of this Japanese patent document is varied, the bond strength of the film can be improved to some degree, but its penetration resistance can scarcely be improved. Furthermore, even if for the same purpose the flexibility of the film itself is increased by increasing the amount of the plasticizer. Its mechanical strength is reduced and sufficient penetration resistance cannot be obtained. Furthermore, if a large amount of the plasticizer is added, it will bleed out in the space between the interlayer film and the glass sheet, and may lead to a reduction in bond strength on long-term use.
In view of the defects of conventional interlayer films of laminated safety glasses, the present inventors made extensive investigations about a resin composition containing a vinyl chloride resin which has versatile application. These investigations have led to a novel composition for use as an interlayer film of a laminated safety glass, which has superior penetration resistance required of a laminate and excellent bond strength and handleability in bonding.